Devices of this general description are shown in WO 95/35126 and EP-A-0 516 473 and tend to employ a drive spring and some form of release mechanism that releases the syringe from the influence of the drive spring once its contents are supposed to have been discharged, to allow it to be retracted by a return spring.
Often, such injection devices are required to work with glass pre-filled syringes that were originally designed for manual use. Such glass syringes have a flange at their base to allow a user to grip the syringe and a needle through which the contents of the syringe can be ejected. Prior to use, the needle is generally covered with a needle shield which may be of plastic or rubber material. The needle shield itself may be contained in a rigid housing which is gripped in a cap on the injection device. Thus, when the cap of the injection device is removed by a user, the needle shield is also removed allowing the device to be operated to extend and expose the needle. The needle shield acts to protect the needle from mechanical damage and maintain its sterility.
In practice, the syringe may not be held rigidly in place within the injection device due, for example, to manufacturing tolerances in the syringe and injection device. In particular, the syringe may be able to move rearwardly in the injection device, i.e. away from the exit aperture. Since the needle shield is gripped in the device cap which is held rigidly in place on a front end of the injection device, if the device is dropped or subjected to adverse external loading, the syringe may move rearwardly so that the needle shield becomes detached from the syringe needle. This is undesirable because the needle is exposed to an environment which may not be sterile. The needle may also become damaged without the protection of the needle shield.